1. Field of the Invention
This invention relates broadly to improvements in semiconductor integrated circuits, and more particularly to improvements in high supply voltage protection for integrated circuits, and still more particularly to improvements in high voltage protection for integrated circuits intended for use in environments such as in automotive applications, and to methods for achieving such protection.
2. Description of the Prior Art
Integrated circuits, and particularly integrated circuits used in automotive applications, are often required to withstand potentially destructive high voltage power supply transients which are greater in amplitude than those normally encountered. For example, in automotive applications, high voltage power supply transients often occur on the battery line which are significantly higher than the normal DC operating voltages of the alternator of a car or other vehicle in which they are employed. This is especially true for integrated circuits which are powered directly from the battery line. These high voltage battery line transients are often referred to as "load dumps". A "load dump" may occur, for example, in the event that the battery becomes temporarily disconnected from the electrical circuit of an automobile of which the motor and alternator system is operating.
In general, it has been assumed that the fabrication process for an integrated circuit of particular use would define a maximum load dump voltage in terms of the breakdown voltage designs of the process, and, typically, integrated circuits sold, for instance to automotive companies, are tested to insure they meet the required breakdown voltage requirements necessary to survive preestablished load dumps. It will be apparent that each particular application will determine the peak voltage and total allowable energy transfer which can be delivered to the integrated circuit. For instance, one such low voltage (60 volts) test which may typically be used is shown in FIG. 1.
As shown, a test circuit 10 has two current loop paths 11 and 12. The first current loop path 11 includes a 60 volt power supply 15, which charges a capacitor 16 through a current limiting resistor 18, when switch 19 is in a first position.
When the switch 19 is switched to a second position, the capacitor 16 is connected into the second current loop 12 to discharge the energy stored on it into the second current loop 12. The second current loop 12 includes a second power supply 21, connected in parallel with the capacitor 16 via a diode 22. The current flows through a resistor 25 to the device under test 26. A second resistor 28 is connected in parallel with the device under test, across which the supply voltage and transient voltages are developed.
Thus, in operation, the voltage provided by the second power supply 21 and diode 22 emulates the steady state voltage, for example provided by an automotive alternator. When the energy stored upon the capacitor 16 is delivered into the second current path 12 upon switching the switch 19, the normal voltage and transient energy are developed across resistor 28 and applied to the device under test 26.
In such 60 volt load dump testing, it can be seen that the process must be designed to produce parts which have a breakdown voltage sufficiently high to assure part survivability. This is usually not a problem in processes for making power integrated circuits. However, absorbing the load dump energy for the 60 volt case is not practical, due to the small source impedance seen by the device under test. A simple calculation shows that the energy stored in the capacitor is 1/2CV.sup.2. Under these circumstances, it is apparent that the process can be designed easily to insure that junction breakdown will not occur.
The problem arises, however, when the load dump voltage rises to a value above the breakdown voltage designed in the process to a level at which the junction will break down, typically between 70 volts and 120 volts. Thus, at those voltage levels, if a load dump of high enough energy were to be applied to the circuit, internal junction breakdown would occur. It should be noted that although this junction breakdown is in itself not destructive, the heating effects related to the breakdown can be destructive, and unless the source impedance seen by the device is large, destruction will likely occur. Furthermore, the ability to control which junctions absorb the high voltage load dump energy is unpredictable.